LOS ANGELES – Arms flexed in a muscleman pose, Aaron Shannon Jr. was getting ready for a Halloween party while his grandfather snapped photos of him in a Spider-Man costume.

Aaron Shannon Sr., left, is comforted by his father, William, during a police news conference in Los Angeles Friday. Two alleged gang members were arrested Friday in 5-year-old Aaron Shannon Jr.’s killing.

Aaron Shannon Jr. was shot while showing off this Halloween costume in his backyard in Los Angeles on Oct. 31. The family’s home is in the heart of gangland, but no one in the family is connected to a gang.

Suddenly, the click of the camera lens was replaced by the pop, pop of gunfire, and the 5-year-old boy was shot in the head.

The Oct. 31 attack blamed on misdirected gunfire from suspected Crips gang members raiding Bloods turf — harkened back to the carnage of the 1980s and early ’90s when brazen young men patrolled the streets of the area once called South Central and gave little thought to living or dying.

Then, Aaron’s death likely would have aggravated the Crips-Bloods rivalry even further and been followed by a retaliatory shooting, then another and so on.

“In the old days, this would have been a massive bloodbath,” said Guillermo Cespedes, head of the city’s gang reduction program. “An incident like this, even a couple of years ago, would have created many more days of violence.”

But it hasn’t. Immediately after the shooting, at least a half-dozen city-funded gang interventionists, experts who are often former gang members, and other volunteers hit the streets in a bid to prevent retaliation.

Residents incensed by the killing of a child were quick to provide details to police, who Friday announced the arrests of Marcus Denson, 18, and Leonard Hall, 21. Both are alleged members of the Kitchen Crips, which for years has been warring with a subset of the Bloods known as the Swans.

Deputy Chief Patrick Gannon estimated as many as 15 additional shootings were stopped.

The boy’s shooting and the days that followed have served as both a reminder of the strife that is all too common in the hardscrabble neighborhoods of South Los Angeles, and a sign of how much has changed.

The alley wall behind Aaron’s house is covered in gang graffiti, and several residents say they regularly hear gunfire and live in fear.

Aaron was standing with friends and relatives in the neatly kept yard of his family’s yellow stucco duplex when the shooting occurred. The bungalow is in the heart of gangland, but no one in the family is connected to a gang.

One block from the house is South Central Avenue, a busy corridor leading to downtown that cuts through industrial areas and impoverished communities of auto-repair shops and low-slung food marts. In Aaron’s neighborhood, the road bisects Crips and Bloods territories.

Javi Ramirez, who owns a carwash at the end of Aaron’s road, said there’s a persistent threat of violence and gang members occasionally take shots at each other across South Central Avenue.

“I’m losing a lot of customers,” he said. “Gang members don’t care if they hit anyone else.”

Police are unsure of a motive, but say two men walked up the alleyway behind the house and one of them fired about a half-dozen shots when he saw the group assembled in the yard. Aaron was struck in the head and died the next day.

“He was the sweetest little boy you would ever meet,” said Ralph Shannon, an uncle, who struggled to hold back tears.